Directed beams of light are used for both measurement and treatment of patients, and the patient presents a moving subject. In the field of ophthalmology, laser photocoagulation is an example of a treatment method, and optical coherence tomography an example of a measurement method, both of which are typically performed with the patient awake, and both of which require precise placement of the light beam on a portion of the eye.
A typical patient can comfortably hold his eye open for a few seconds. The eye moves considerably in one second, mainly through quick adjustments in fixation (small saccades) resulting in apparent motions of the retina on the order of one hundred microns. These motions cause noticeable errors in application of directed beams such as photocoagulation and optical coherence tomography (OCT). Tracking the motion of the eye to correct the placement of the beam has proven useful in photocoagulation [Naess, E., et al. (2002)] and in OCT [U.S. Pat. No. 6,736,508; Hammer, D. X., et al. (2005)].
Typically, a pair of rotating mirrors serves as a two-dimensional scanner to move the beam of light in two dimensions, x and y, across the subject. If motion of the subject is tracked, the positioning commands to the scanner can be adjusted so that the scan beam reaches at the desired positions on the subject.
Information on the motion of the subject must be provided with low latency so that the scanning beam is correctly positioned for each A-scan in OCT, or for each laser shot in photocoagulation. In a system that corrects the scan beam position, the latency is the time between eye motion and correction of the position of the scan beam.
Tracking methods that use two-dimensional image frames [U.S. Pat. Nos. 4,856,891; 5,729,008; 5,975,697; and U.S. Patent Application Publication No. 2005/002458] have the advantage that the two-dimensional image can also be used for a real-time display to orient the operator during the measurement or treatment procedure. These methods typically incur latency approximately equal to the time between frames, which is typically 1/30 of one second. During one frame, the eye can move significantly [Hammer, D. X., et al. (2002)]. Faster frame rates are possible, but incur extra cost.
Tracking methods that use a dithered tracking beam are fast enough to follow the motion of a human eye [Hammer, D. X. et al. (2002); U.S. Pat. Nos. 5,943,115, 5,767,941]. Dithered-beam methods with update rates of 2-10 kHz have been successful in tracking the human eye. The dithered tracking beam requires a separate optical scanning system, in addition to the system used to scan the treatment or measurement beam.
A line-scan ophthalmoscope (LSO) produces an image of the eye one line at a time [U.S. Pat. Nos. 4,732,466; 4,768,874; and 6,758,564]. In an LSO using an electronic camera, each line can be acquired and made available to digital processing within less than one millisecond. The part of the eye image contained in each line can be compared to the same area in previous eye images in order to determine the eye motion. Individual lines from an electronic LSO are available at approximately 10 kHz.
Previously disclosed tracking methods typically use a landmark, such as the optic disk. The landmark is identified first, and its location is monitored as the measurement, or treatment, scan proceeds. However, good landmarks are not always found in diseased tissue.
We see a need for a system to track motion of the eye, or other human tissue, with low latency during an optical treatment or optical measurement procedure, where the tracking system shares apparatus with a system providing a real-time display to the operator, and using a method that is independent of any particular landmark in the tissue.